Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Need help with portable 20m dipole  (Read 1128 times)
KJ4JQU
Member

Posts: 35




Ignore
« on: May 14, 2013, 08:22:22 PM »

Hello all, thx for reading this. I'm building a simple 20m dipole for use on vacations. My issue is finding the best " compromise" as far as length. By simple I mean simple. It's bare stranded wire, 50ohm coax directly soldered to it. Insulators on the ends and the middle. The first time I'm actually going to set it up will be at the beach in a sloped position facing the ocean. Who know where else it may get set up-'perhaps as a flat top- maybe a vee- again who knows. As for dimensions I have seen that based on set up and what's around it, your supposed to trim to get the best swr. But what would be the best length to kind of set and forget it as it will be configured multiple ways. I'll be running it @ 100watts- I do have an ant tuner. Thanks for any suggestions!
Logged
WB6BYU
Member

Posts: 12773




Ignore
« Reply #1 on: May 14, 2013, 08:47:41 PM »

I built my portable antenna kit over 30 years ago, using #22 stranded, insulated hookup wire
and RG-174 coax.  I took it to the local park and adjusted each band with the feedpoint up
about 12' and the ends at 4' or so.  That's not a very good installation, but I chose it as a
compromise:  when it is high in the air the SWR may be a bit worse, but then it will be
working better.

Since then I've used the kit half way around the world, from Australia to Nova Scotia and
Alaska to New Zealand, as well as from a number of states.  When operating out of a backpack
I generally don't carry a tuner - just set it up and operate.

So I'd recommend that you choose a similar configuration, prune it to length, then just use it.
The SWR may be higher in some situations, but it will still work.

The exact resonant length depends on the wire diameter, insulation type, height above ground,
how you tie the strings on the ends, etc., so it is difficult to recommend a specific length without
adjustment.  I'd probably start at about 17.5 to 18 feet on each side, tie the support ropes about
a foot in from each end so the tails hang down, then trim the tails a little at a time for minimum
SWR.


My first backpack antenna used solid magnet wire, but after one trip I switched to thin stranded
insulated wire because it was more flexible and less prone to kinking.  I can tie knots in it to
join the ropes or to the center insulator, etc.  #26 wire is fine for light weight backpacking
antennas, while I might go with #22 or #18 where weight and space aren't limitations.
Logged
W1JKA
Member

Posts: 1390




Ignore
« Reply #2 on: May 15, 2013, 06:03:59 AM »

BYU covers just what you need,I do the same for 20,30,40 meter radiators.The only thing I do different is pretune them in my backyard at 15 ft. at an angle halfway between horizontal and my usual inverted Vee configuration.My general purpose heights are between 10-20 ft.and I vary configurations.By this mid range tuning method (right or wrong) my rigs work just fine in different antenna configurations without having to mess with a tuner/swr meter during my portable ops.The nice thing especially on 20m is the easy change of directivity with either Vee or horizontal.
Logged
KJ4JQU
Member

Posts: 35




Ignore
« Reply #3 on: May 15, 2013, 06:41:59 AM »

Ok- well those measurements are a little different then what I had read. Guess I was shooting for closer to 33 feet. Right now one end is 16.7 and the other is 16.9. Is it a huge deal that both sides are perfectly equal?  Part of the reason I posted this was I kept reading that folks had huge changes in swr when they were a 1/4 in off target length. Has me worried that without extreme precision performance will be terrible. As stated I have a tuner but as we all know just because the radio likes what it's seeing doesn't mean that the signal is getting out any better lol
Logged
W5DXP
Member

Posts: 3391


WWW

Ignore
« Reply #4 on: May 15, 2013, 07:02:55 AM »

I have a tuner but as we all know just because the radio likes what it's seeing doesn't mean that the signal is getting out any better

If your coax is reasonably short, I wouldn't worry about any SWR lower than about 6:1 which you can measure by putting your tuner in bypass mode. OTOH, if your tuner is an internal tuner inside your transceiver, you probably need to trim the antenna so the SWR is less than 3:1.
Logged

73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
W1JKA
Member

Posts: 1390




Ignore
« Reply #5 on: May 15, 2013, 07:13:56 AM »

Re:KJ4JQU

Reading is fine,but for best results just cut,do and find out for yourself.
Logged
KJ4JQU
Member

Posts: 35




Ignore
« Reply #6 on: May 15, 2013, 07:14:27 AM »

External tuner, sorry should have specified. But it's not a great one. It's an older MFN that was designed for use in a car I think. Anyway thanks for the tips so far!
Logged
AC2EU
Member

Posts: 301


WWW

Ignore
« Reply #7 on: May 15, 2013, 07:18:37 AM »

Ok- well those measurements are a little different then what I had read. Guess I was shooting for closer to 33 feet. Right now one end is 16.7 and the other is 16.9. Is it a huge deal that both sides are perfectly equal?  Part of the reason I posted this was I kept reading that folks had huge changes in swr when they were a 1/4 in off target length. Has me worried that without extreme precision performance will be terrible. As stated I have a tuner but as we all know just because the radio likes what it's seeing doesn't mean that the signal is getting out any better lol

33 feet is "mid band" around 14.175 MHZ. At 20 meters, 16 to 17 feet up is quarter wave so that should be easy to accomplish as a portable installation. Mod band is a good choice if you do phone and CW. The rule of thumb is that a 1/4 wave center fed dipole at resonance is about 72 ohms. I prefer making portable dipoles with #14 THHN insulated wire for flexibility and ruggedness. the insulation does affect the resonance, but not that much. The swr is affected by the feed point, but not noticeably moving a quarter inch at 20M! Maybe at 70 cm... The change in resonance is a linear function of length.

The feed cable should be a half wave electrical length or better at the LOWEST frequency used, say about 23 feet using a VF of .66.
The antenna and the feed line are a SYSTEM . the two together determine what impedance is presented to the radio.
People who report odd swr issues probably have not taken this into consideration.

Borrow an antenna analyzer if you can. It's the best way to be sure that you have the system working correctly. It's also very educational.

 
Logged

KJ4JQU
Member

Posts: 35




Ignore
« Reply #8 on: May 15, 2013, 07:50:04 AM »

I'm only a general so the lowest I can talk is 14.225. Don't do CW. Coax feed line is 40ft but I did a 4in 6 turn in it right under the center insulator as choke just in case-(heard its a good idea). Don't have access to an analyzer though I'm sure that would be a great tool. Don't really want to play with trimming too much- especially as it will be set up different ways every time I use it. With this added info, is there not a general solid measurement to use- knowing full well it will be a compromise- that is both safe for the equip and people around it- while also being very effective. Is the way it's trimmed now ok? That's really what I'm trying to figure out.good to know 1/4in won't throw it off too bad. Sorry if that's comes across as unappreciative but I don't have any other tools and since ill be on vacation with family from Europe my wife will have little patience for me spending hours trying to tune an antenna just because I'm bored at the beach. She's already pissed that I don't like the beach!! Thanks again
Logged
WB6BYU
Member

Posts: 12773




Ignore
« Reply #9 on: May 15, 2013, 08:15:13 AM »

Try 16.5 feet each side, for a total length of 33'.  That should get you pretty close, but
again it depends on some of the construction details.  That's why we are recommending
that you tune it once before you leave (just to check that the SWR is low in the desired
band) and then don't bother trying to tune it while you are on vacation.

As a gauge of sensitivity to length, EZNEC suggests that lengthening the total antenna
length by 1 foot will lower the resonant frequency from 14.3 to 13.9 MHz, and increase
the SWR at 14.3 MHz to about 2.4 : 1.  That's well within the range that you can match
with a tuner with no significant increase in feedline loss.

It also gives you an idea that trimming 1.5" off each end will move the resonant frequency
up by about 100kHz.  (You can assume this is linear in the vicinity of 20m, but it won't be
the same on other bands.)

While your dipole is probably OK as it is, I'd suggest at least adjusting the two legs to be
the same length, even though it might not make a lot of difference.  Beyond that, don't
overthink the problem.  And if one wire breaks, twist it back together and don't worry
about the difference in length.  Just enjoy operating.


And if you are at the beach, try stringing the dipole vertically or as a sloper with one end
elevated, especially if you can find a convenient tree that overhangs salt water.  Vertical
polarization over salt water is amazingly effective for DX.
Logged
W1JKA
Member

Posts: 1390




Ignore
« Reply #10 on: May 15, 2013, 08:50:13 AM »

Don't forget the UMBRELLA,that along with a simple 20m dipole you will soon learn to love the beach.
Logged
AC2EU
Member

Posts: 301


WWW

Ignore
« Reply #11 on: May 15, 2013, 08:53:29 AM »

I'm only a general so the lowest I can talk is 14.225. Don't do CW. Coax feed line is 40ft but I did a 4in 6 turn in it right under the center insulator as choke just in case-(heard its a good idea). Don't have access to an analyzer though I'm sure that would be a great tool. Don't really want to play with trimming too much- especially as it will be set up different ways every time I use it. With this added info, is there not a general solid measurement to use- knowing full well it will be a compromise- that is both safe for the equip and people around it- while also being very effective. Is the way it's trimmed now ok? That's really what I'm trying to figure out.good to know 1/4in won't throw it off too bad. Sorry if that's comes across as unappreciative but I don't have any other tools and since ill be on vacation with family from Europe my wife will have little patience for me spending hours trying to tune an antenna just because I'm bored at the beach. She's already pissed that I don't like the beach!! Thanks again

Once you set the system up, you can make notes about height vs resonance and impedance so you will know more or less what to expect, barring any nearby things that it may couple to.
Your "mid band" is 14.288, so total length would be approx 32 feet, 9 inches depending on the wire used and center fed. Your present configuration is too long for your purposes. The swr curve will be better if you center it on the frequencies where you intend to use it. (less BW to contend with)  No tuner should be necessary.

BTW, I can only do about two days MAX on the beach before the boredom ruins my vacation. What is it with women and laying on beaches for days on end, anyway?  Huh Good luck!
Logged

KJ4JQU
Member

Posts: 35




Ignore
« Reply #12 on: May 15, 2013, 11:40:05 AM »

Ok great thanks! Yea I hate the beach- it's hot- there is sand everywhere- usually involves an 8+hr drive. The only redeeming quality is bikinis- but for every good one of those there's 10 you really never wanted to see! Some things you can't unsee ya know!
Logged
K1WJ
Member

Posts: 449




Ignore
« Reply #13 on: May 15, 2013, 01:33:12 PM »

468/14.250 will get you in the ball park = 32.84ft, then divide by 2 for each leg = 16.42ft
73 K1WJ
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!